Quantitative analysis of light-induced ion segregation in mixed-halide perovskites

Mixed-halide perovskites (MHPs) offer good band-gap tunability via stoichiometry changes, and such tunability is an essential property for the creation of multijunction solar cells. However, under illumination halide ions in MHPs segregate and create I- and Br-rich regions, which decrease the efficiency of potential solar cells. In this work, a method for a detailed investigation of the distribution of halide ions within an MHP during and after illumination is introduced. Calculations of the strain field created by the halide segregation were performed, and the obtained local displacement of atoms was used to calculate the X-ray diffuse scattering. By fitting the experimental data measured on a thin polycrystalline layer of FA_0.83Cs_0.17Pb(I_0.6Br_0.4)₃ (where FA stands for formamidinium), the distribution of Br^- and I^- ions within an illuminated MHP was determined and the subsequent relaxation process of the segregation in the dark was tracked. The creation of highly Br-rich regions within a slightly I-rich volume during illumination was observed.